The present invention relates to the construction of a package for an optical fibre Bragg grating so as to facilitate accurate setting of the centre frequency. The preferred embodiment including a package which provides a stable centre frequency at a range of temperatures.
Bragg gratings are well known in the field of optical fibre construction and normally comprise a repeating pattern written into a photosensitive optical fibre by a UV light source or the like unfortunately, the modulating pattern is normally inherently highly sensitive to the effects of strain and temperature. This is a disadvantage in that in applications, such as communications or sensing systems, it is often a requirement that the optical characteristics of the grating are not influenced by the effects of temperature or strain. For this reason, it is a typical requirement to mount the grating in a package so as to isolate the grating region from external strain or temperature effects.
In FIG. 1, there is shown a simple grating package 1 which mounts a fibre 2 at two ends of a package mount 3 by means of fixing with epoxy 4. Unfortunately, with this arrangement expansion of the package 3 with temperature relative to the fibre 2 will result in a change in tension on the fibre 2.
In FIG. 2, there is shown a slightly more complex arrangement 10 which includes a temperature compensating package which uses a combination of materials with differing thermal expansions. A first material 11 is utilised to expand at a first low rate while a second material comprising end portions 12, 13 expands with temperature at a substantially higher rate. The combined expansions of the two forms of material 11, 12 and 13 produces a more stable arrangement whereby the fibre fixing points 15, 16 are maintained thereby maintaining the overall strain on the fibre 18 in the intermediate region so as to counteract the effect of temperature on the centre frequency.
In some applications, the temperature characteristic may not require specific compensating. However, it may be the case, such as when utilising the arrangement 1 of FIG. 1, that the fibre is still required to be packaged under strain so as to achieve a greater wavelength accuracy than that which can be achieved in the fibre writing process.
In the majority of packaged Bragg fibre gratings, the fibre is held under strain. When fixing the fibre to the package, a number of techniques may be employed. These include epoxy adhesive or glass xe2x80x9cfrittingxe2x80x9d. These processes often require heat and as a result, they can have a further effect on the wavelength of the grating making it difficult to distinguish between the effects of strain or heat when setting the fibre in the package and setting the corresponding wavelength. Further, typical methods for fixing the fibre to the package may have some shrinkage effects as either the epoxy cures or the glass frit sets. The shrinkage can further introduce unwanted changes to the strain on the fibre which result in an alteration of the wavelength setting.
Further, the materials utilised often require that an epoxy resin or the like is used to affix the fibre to the package. This can induce the problem of creep wherein, over time, the characteristics of the wave guide change due to creep of the adhesive material relative to the package or to the fibre. Unfortunately, epoxy can be undesirably humidity sensitive. The unavoidable use of an epoxy is often required to be utilised due to the necessity to maintain compatibility of materials.
It is an object of the present invention to provide an improved temperature stable optical fibre package.
In accordance with a first aspect of the present invention, there is provided a method of accurately setting the centre wavelength of a fibre grating fixed in a temperature compensation package, the method comprising the steps of fixing the grating to the package and altering the size of the package in a post processing step.
The altering step can comprise elongating or compressing the package so as to substantially permanently alter the size of the package.
The temperature compensation package can comprise a first member having a first coefficient of thermal expansion; two second members, one each attached to the ends of the first member and having their other second ends closer together then the ends of the first member, the second members having a second coefficient of thermal expansion relatively more then the first coefficient of thermal expansion; and two material matching members attached to the second ends and adapted to provide for material matching fixing mixtures to the second members; said fixing mixtures fixing ends of the fibre grating to the temperature compensation package.
The fixing mixture can comprise glass or metal solder.
The material matching members are preferably located within an aperture in the second ends.
In accordance with a further aspect of the present invention, there is provided a temperature stable grating package having an accurately fixable centre frequency, the package including: fixing means for fixing a grating fibre to the package; temperature compensation means for compensating for any fluctuations in temperature of the package; and package length alteration means for altering the length of the package so as to accurately and substantially fix the centre frequency of the grating.
The temperature compensation means can comprise: a first member having a first coefficient of thermal expansion; two second members, one each attached to the ends of the first member and having their other second ends closer together then the ends of the first member, the second members having a second coefficient of thermal expansion relatively less then the first coefficient of thermal expansion; and two material matching members attached to the second ends and adapted to provide for material matching fixing mixtures to the second members; and the fixing means fixing ends of the fibre grating to the temperature compensation package.
The fixing means comprise glass or metal solder and the material matching members can be located within an aperture in the second ends.
In accordance with a further aspect of the present invention, there is provided a temperature stable grating package including: fixing means for fixing a grating fibre to the package; temperature compensation means for compensating for any fluctuations in temperature of the package; wherein the temperature compensation means can comprise: a first member having a first coefficient of thermal expansion; two second members, one each attached to the ends of the first member and having their other second ends closer together then the ends of the first member, the second members having a second coefficient of thermal expansion relatively less then the first coefficient of thermal expansion; and two material matching members attached to the second ends and adapted to provide for material matching fixing mixtures to the second members; and the fixing means fixing ends of the fibre grating to the temperature compensation package.
The fixing means can comprise glass or metal solder.